Cable television systems, or CATV systems, are used in a widespread manner for the transmission and distribution of television signals to end users, or subscribers. In general, CATV systems comprise a transmission subsystem and a distribution subsystem. The transmission subsystem obtains television signals associated with a plurality of CATV channels and generates a broadband CATV signal therefrom. The distribution subsystem then delivers the CATV broadband signal to television receivers located within the residences and business establishments of subscribers.
One problem facing CATV service providers is signal leakage. Signal leakage refers to the unintentional transmission and/or reception of signals through breaches or other nonconformities in the CATV distribution subsystem. In particular, the distribution subsystem, which typically comprises coaxial cable, amplifiers and other devices, ideally provides a relatively low-loss conduit between the CATV transmission subsystem and subscribers' television receivers. If, however, portions of the distribution subsystem are physically damaged, for example, the coaxial cable is damaged, kinked or broken, then CATV signals may leak through the damaged portions, causing unwanted transmission into the atmosphere.
The primary problem associated with the transmission of the CATV signal into the atmosphere via leakage is potential interference with aeronautical communications. Portions of the allocated CATV bandwidth overlap with frequencies allocated for aeronautical communication. Excessive leakage of CATV signals can therefore undesirably interfere with aeronautical-related signal transmission and reception.
As a result, CATV service providers strive to reduce the undesirable phenomenon of signal leakage.
The first step to reducing signal leakage is to determine the location of leakage points in the distribution subsystem. Various leakage detection devices are currently available that assist in the location of leakage points. Such devices typically include an antenna and a receiver that is tuned to a particular frequency in the CATV signal bandwidth. The detector further includes a signal strength measurement circuit. To detect leakage, a technician typically drives along a route that traces a portion of the CATV distribution system, preferably in the vicinity of a suspected leakage location. If the signal strength measurement circuit detects a relatively large amplitude signal at a particular location, then a leak may be indicated in or near that location. The technician may then use the leakage detector to pinpoint the source of the leak. Once the source of the leak is pinpointed, corrective action may be taken.
A drawback of the above described leakage detection devices is their inability to distinguish CATV signals leaked from the system under test from other signals in the same bandwidth. This drawback is becoming of increasing importance due to the proliferation of CATV service providers. In particular, two or more CATV service providers often have portions of their distribution systems that overlap, or at least are disposed in close proximity to one another. As a result, when a technician detects an RF signal in a particular location, the detected RF signal may either have originated from leakage in the system under test or from leakage in another system. Because CATV service providers are primarily interested only in leakage in their own distribution system, it is desirable to ascertain the identity of the source of the leak.
One prior art method of addressing the problem of differentiating the leakage signals from a system under test from other signals is described in U.S. Pat. No. 4,237,486 to Shimp, issued Dec. 2, 1980. Shimp describes a method of modulating an audible tone on an unused CATV channel frequency carrier at the transmission subsystem. The audible tone is sometimes referred to as a tagging signal. The leakage detector, which is tuned to the CATV channel frequency, receives and demodulates the signal. The demodulated signal is audibly amplified. If the audible tone is present, then a significant amount of leakage in the system under test is indicated. If, however, no audible tone is present, then no leakage in the system under test is indicated. Such a system allows the technician to distinguish between detected RF signals originating from leakage in the system under test, which include the audible tone tagging signal, from RF signals originating from extraneous sources, which will not include the audible tone.
One drawback to the above technique is that it relies upon the user's ear to detect the distinctive audible tone. The drawback to such reliance is that if another CATV service provider is transmitting another distinctive audible tone in a closely located but separate CATV network, the user must distinguish between audible tones to determine whether the leakage signal was leaked from the system under test as opposed to the separate CATV network.
Yet another problem is associated with the Shimp device is that it cannot readily distinguish an absolute measure of the quantity of leakage based on the audible tone. While the leakage detector may be moved from location to location to obtain relative readings through detectable increases and decreases in volume, the Shimp device provides no means for obtaining an absolute level.
To overcome the deficiencies of Shimp, leakage detectors have been developed that determine the relative amplitude of the tagging signal (i.e., the modulated tone) with respect to the amplitude of the detected RF signal. Such leakage detectors determine whether the relative amplitude of the tagging signal is consistent with modulation depth of the tagging signal. In particular, prior tagging signal generators typically modulated the RF carrier signal with a substantially pure (or in other words, harmonic free) sinusoidal signal having a known frequency and further having a predetermined depth of modulation. The frequency of the sinusoidal signal is referred to as the tagging frequency. The leakage detector then uses filtering or digital means to determine the relative amplitude of the tagging frequency component of the detected RF signal with respect to the average amplitude of the detected RF signal. The relative amplitude translated directly to the depth of modulation of the generated RF signal.
Consider, for example, a tagging signal generator that generates a 30 Hz sinusoidal tagging signal and modulates the tagging signal onto an RF carrier signal having a frequency of 115 MHz using a 3 dB depth of modulation. Using the 3 dB depth of modulation, it is known that the sinusoidal tagging signal has a relative amplitude of approximately 17% of the average amplitude of the RF carrier signal. The prior art leakage detector described above would obtain the signal content in a signal band centered around 115 MHz, and then demodulate the 115 MHz signal. The prior art leakage detector would then determine the average amplitude of the entire signal, as well as the relative amplitude of the 30 Hz frequency component of the signal. If the relative amplitude of the 30 Hz frequency component is determined to be sufficiently close to 17% of the amplitude of the entire signal, then it was determined that the detected signal originated from leakage in the system under test. If, however, the relative amplitude of the 30 Hz frequency component was significantly less than 17% of the average amplitude of the entire baseband signal, then it was determined that the detected RF signal did not originate from leakage in the system under test.
A leakage detector that operates on this principle is described in U.S. patent application Ser. No. 89/979,104, to Shi et al., filed Nov. 26, 1997, which is assigned to the assignee of the present invention and incorporated herein by reference.
While such systems are effective for leakage detection operation, it is always advantageous to reduce cost in the system, for example, in the leakage tagging generation equipment. One source of cost is the tagging signal generator, which for several reasons is typically a sinusoidal signal source. One reason a sinusoidal signal source has historically been used in the prior art is that tagging signal generators were intended to modulate the tagging frequency signal onto an active television signal. To reduce interference with video reception of that television signal, the tagging frequency signal was chosen to be a low frequency sinusoid that, while detectable by leakage detectors, was filtered out by the input circuitry of many television receivers. If a non-sinusoidal signal is used, then the higher frequency components of the signal tend to increase interference with television reception.
While such systems generate suitable RF tagging signals for leakage detection, there is still a substantial possibility of interference with the video and/or audio content of the received television signal, even with the use of a low frequency sinusoid tagging signal.
A need therefore exists for a tagging signal generator that eliminates the cost associated with generating harmonic-free sinusoidal tagging signals without increasing the interference with television reception.